At some point in the process of creating paper from a pulp formed from wood or some other substance, it is often desirable to "screen" the pulp. This term is used to mean the filtration of the pulp to rid it of impurities and undesirable fibers. This screening is done by applying the aqueous pulp suspension or slurry against a screen or other filter device. The fiber that is considered acceptable for papermaking passes through the screen and the undesirable fiber is retained on the screen and rejected.
In addition to, or apart from screening, it is often desirable to fractionate pulp. "Fractionation" is used to mean the process by which pulp is sorted into different fiber lengths. The fractions of pulp resulting from fractionation each contain fibers of relatively uniform length. The process of fractionation is identical to that of screening, with one difference: in fractionation, the fibers are sorted by fiber characteristics, rather than accepted or rejected on the basis of desirability.
One way to categorize the screening/fractionation devices (hereinafter "fractionation devices") in the prior art is by the type of force used to pass the pulp suspension through the screen.
One class of fractionation devices uses the force of gravity by placing the pulp suspension on a screen and collecting the acceptable fiber beneath it.
Another class of fractionation devices uses centrifugal force to pass pulp suspension through the screen. A widely used device of this class may be described as follows: a cylindrical screen is used to filter pulp suspension, and a rotatable shaft is located coaxially through the screen. Attached to the rotatable shaft, extending radially, are one or more blades or foils, whose outer edges are very close to the screen. Pulp suspension is introduced into the cylinder defined by the screen and occupied by the rotatable shaft and blades or foils, and the shaft is rotated. As a result of this rotation, the blades or foils attached to the shaft move the suspension in a circular motion, creating a centrifugal force which pulls the pulp suspension outward. The blades or foils entrain the pulp away from the center of the separation chamber and press it against the screen, through which the smaller particles and fibers escape.
In addition to forcing smaller particles and fibers through the screen, the foils prevent longer fibers from escaping end-first, by the creation of a low-pressure area or "shear force" between the edge of the foils and the screen. This shear force has the effect of pulling any undesirable fibers escaping through the screen back toward the shaft. Since longer fibers require more time to escape than shorter ones, their escape is more likely to be interrupted by the passage of a blade or foil, and the consequent pulling effect of the shear force. As the frequency by which the foils pass a given point on the screen ("tip frequency") increases, the less likely it is for a longer fiber to escape successfully. The tip frequency can be altered either by varying the rotational speed of the shaft, or by varying the number of foils or blades on the shaft.
Multiple stages are often desirable in screening as well as in fractionation applications. "Series screening" is often used to more completely purify a pulp suspension. This process consists of screening the suspension through a plurality of similar or slightly dissimilar devices to ensure the complete removal of undesired elements. Multi-stage fractionation is used when more than two fractions are desired.
A need for improved multi-stage fractionation and screening devices ("multi-stage fractionation devices") has arisen with the development of new processes for producing paper pulp. These processes treat the fibers so that the fibers develop "fibrils," small spiral hairs or threads along the fibers' length.
Conventional multi-stage fractionation devices pump the slurry being treated from stage to stage. Such systems therefore require, between the stages, piping, a pump inlet box, a pump, and equipment for level regulation with an automatic valve at the pressure side of the pump. The mechanical working of the fiber, by pumping from one stage to another, causes some fibrils to come loose from the fibers, degrading the quality of the pulp. The more stages in which the pulp is treated, the more the quality of the pulp is degraded.
The loose fibrils themselves also negatively affect pulp quality. They, together with other small particles, form undesirable "fines," which degrade the quality of the pulp and reduce the capacity of subsequent treating machinery, e.g., dewatering equipment.
Conventional processing also reduces the capacity of dewatering equipment by introducing air, through the open inlet box of each pump, which mixes with the pulp.
Additionally, in conventional multi-stage fractionation devices it is difficult to remove, add, or replace stages. Such operations typically require long periods of "down time."